For example, if a user account is identified with a username, there can only be one account with that username. Accounts may eventually be deleted and that username (or other unique identifier) intentionally released for re-use, but that new account may not have the same permissions or access as the first account that was deleted. This doesn't preclude the use of shared accounts (except where it is strictly forbidden, like in-scope PCI systems) and applies to both individual and shared accounts. If a shared account is used, that account must have a unique identifier in the same way an individual, non-shared account does.